This invention relates to an apparatus for optically detecting toner concentration in a powdery developer used for an electrophotographic copying apparatus.
In electrophotographic copying process using a powdery developer composed of toner and carrier, the toner particles could deposit in the area that should become a blank portion of an image if the toner content in the developer is too high. This phenomenon is called "fogging". If the toner content is too low, a resultant image is faded to become unfaithful to the original. Fogging and image-fading not only deteriorate image quality but shorten the life of the developer. It is therefore necessary to keep the toner concentration constant. But, since there are various types of originals which differ in toner consumption, it is hardly possible to maintain the toner concentration constant merely by replenishing toner at constant rate.
To solve the problem, various apparatus have been proposed for controlling toner concentration. The apparatus are provided with a detector for detecting toner concentration in a developer electrically, magnetically or optically. The detector produces a detection signal, in accordance with which a proper amount of toner is automatically replenished. An optical toner concentration detector, however, has its sensitivity gradually lowered because toner particles enter it and stick to its light-emitting and light-receiving elements. To have its sensitivity maintained to a satisfactory degree, the optical toner concentration detector should often be cleaned. The cleaning, however, is troublesome and time-consuming.